1. Field of the Invention
The present invention relates to a support unit, an optical unit and an exposure apparatus, and a device manufacturing method, more particularly to a support unit that supports light transmissive optical members arranged on an optical path of illumination light, an optical unit that comprises a barrel equipped with the support unit, an exposure apparatus using the optical unit, and a device manufacturing method that uses the exposure apparatus for manufacturing devices.
2. Description of the Related Art
Conventionally, various exposure apparatus have been used in a lithographic process for producing electronic devices such as a semiconductor device (integrated circuit) and a liquid crystal display device. Ones mainly used in recent years are projection exposure apparatus such as a reduction projection exposure apparatus (a so-called stepper) based on a step-and-repeat method, which reduces and transfers a pattern of a mask (also called a reticle), which is formed by proportionally enlarging a pattern to be formed about four to five times, onto a substrate to be exposed such as a wafer via a projection optical system, and a scanning projection exposure apparatus (a so-called scanning stepper(also called a scanner)) based on a step-and-scan method, which is an improvement of the stepper.
With these projection exposure apparatus, exposure wavelengths have shifted to a shorter wavelength range in order to achieve high resolution to cope with finer circuit patterns that goes with higher density and higher integration of integrated circuits. Specifically, instead of the emission line (such as g-line and i-line) of a mercury lamp that has been mainly used conventionally, a KrF excimer laser having a wavelength of 248 nm is mainly used as an exposure wavelength at present, and the usage of an ArF excimer laser having a shorter wavelength of 193 nm is also entering a practical stage. Further, proposals are recently being made on projection exposure apparatus using an F2 laser having a shorter wavelength of 157 nm or an Ar2 laser having a shorter wavelength of 126 nm.
Such ultraviolet light having a wavelength of 190 nm or shorter is called vacuum ultraviolet light, and the vacuum ultraviolet light is strongly absorbed by almost all substances. For example, oxygen, water vapor, carbon dioxide, and almost all typical organic substances strongly absorb the vacuum ultraviolet light. Accordingly, in an exposure apparatus that uses vacuum ultraviolet light as its exposure light, the above-described light absorbing substances such as oxygen, water vapor, carbon dioxide, and organic substances need to be reduced or removed from the optical path where the exposure light passes in order to allow the exposure light to reach a wafer surface with sufficient illuminance. Generally, in order to perform manufacturing operation of electronic devices while securing sufficient productivity (throughput), the concentration of gas containing impurities such as the above-described light absorbing substances (hereinafter referred to as “absorptive gas”) is removed from the space of the optical path to several ppm or less and the space of the optical path needs to be replaced with gas that absorbs only a small amount of vacuum ultraviolet light such as rare gas like nitrogen or helium (hereinafter referred to as “low absorptive gas”).
Further, even when ultraviolet light having slightly longer wavelength (about 193 nm) than that of the vacuum ultraviolet is used, oxygen gas is preferably removed from the optical path since a small amount of the ultraviolet light is absorbed by oxygen gas.
As a replacement gas (purge gas) that is to be supplied into the optical system of the exposure apparatus, an illumination optical system, and the barrel (or housing) of a projection optical system, rare gas such as nitrogen (N2) and helium (He) can be given, and of the gases, helium gas is extremely effective for suppressing an increase in temperature which occurs when lenses or the like that are components of an optical system absorb the exposure light, that is, from the viewpoint of cooling effect.
However, the refractive index of helium gas (about 1.000038) greatly differs from the refractive index of normal air or nitrogen (about 1.000319). Therefore, when nitrogen, air or the like enters a space filled with helium gas from the outside, the refractive index inside the helium gas space greatly changes, which causes aberration and desired optical performance cannot be realized.
In this case, a configuration is possible where each gas space is partitioned with the use of various optical members such as lenses arranged in the optical path space of the exposure apparatus. However, for adjustment of optical properties of the optical system, the position and the attitude of the optical members such as lenses need to be adjusted in most cases, and a clearance needs to be secured around the optical members with an extent in which the movement of the members is not interfered. However, because the clearance becomes a flow path for gas, it is difficult to completely prevent the gas entry described above.
Alternatively, to partition optical path spaces having different gas types, it is possible to install an optical member (hereinafter referred to as “partition wall optical member”) that serves as a partition wall to stop the gas movement. However, since it is often the case where the partition wall optical member is arranged in an optically important position, the above-described clearance for adjustment needs to be provided in the same manner as the above-described optical members such as the lenses, and there is a risk that the gases will enter the space via the clearance.
To prevent the gases from entering the space, it is possible to completely block the gas movement by using sealing members such as an o-ring, which is often used in a vacuum unit, and various gaskets, without providing the above-described clearance for adjustment. However, when such a method is used, the partition wall optical member sandwiched in between the sealing members normally receives great pressure, and the partition wall optical member may be deformed significantly. Then, the wavefront of the exposure light having passed the deformed partition wall optical member is deformed significantly, which generates various optical aberrations (wavefront aberrations), which in turn keep the exposure apparatus from realizing a desired optical performance. This has been proven experimentally.
Furthermore, other than the sealing members used in the vacuum unit, a sealing method by a v-ring, an irregular o-ring, or the like, highly viscous fluid such as oil, or high-pressure fluid may also be considered. However, in these methods, it has been experimentally clarified that it is difficult to realize high-purity purge of the ppm level required by an exposure apparatus having the F2 laser as a light source under the same atmospheric pressure environment as the optical path space of this type of exposure apparatus.
Furthermore, when fluid such as grease is used, it is possible to satisfy conflicting requirements such as high-purity purge of ppm level and suppressing the load (pressure) to the partition wall optical member. However, grease or the like usually emits gases such as organic substances. Such emission gases were also regarded as a problem in an exposure apparatus having the ArF excimer laser as a light source, where photochemical reaction between such gases and the exposure light occurs, causing them to deposit on the optical members and absorb the exposure light, generating a so-called “clouding”. This makes it difficult to supply light having sufficient illuminance on a wafer surface, which makes this method a less than practical measure.
Specifically, in the case of holding the optical members that serve as the partition wall, at least the following four points must be simultaneously satisfied.                (1) Realize high-purity purge of ppm level and suppress the mixture of gases in two spaces partitioned by the partition wall optical member.        (2) Minimize the load (pressure) to the partition wall optical member as much as possible.        (3) Prevent gas emission that causes light absorption.        (4) Keep high installation reproducibility when exchanging optical members.        